Surprisingly Effective Priming of CD8⁺T Cells by Heat-Inactivated Vaccinia Virus Virions

Abstract

Robust priming of CD8+ T cells by viruses is considered to require infection and de novo expression of viral antigens. A corollary of this is that inactivated viruses are thought of as being inevitably poor vaccines for eliciting these responses. Contrary to this dogma we found that some antigens present in vaccinia virus (VACV) virions prime strong CD8+ T cell responses when the virus was rendered non-infectious by heat. More surprisingly, in some cases these responses were similar in magnitude to those primed by infectious virus administered at an equivalent dose. Next we tested whether this was a special property of particular antigens and their epitopes and found that foreign epitopes tagged onto three different VACV virion proteins were able to elicit CD8+ T cell responses irrespective of whether the virus was viable or heat-killed. Further, the polyfunctionality and cytotoxic ability of the CD8+ T cells primed by these VACVs was equivalent irrespective of whether they were administered to mice as inactivated or live viruses. Finally, we used these VACVs in prime-boost combinations of inactivated and live virus and found that priming with dead virus before a live booster was the most immunogenic regime. We conclude that VACV virions themselves can be efficient vectors for targeting antigens to dendritic cells for effective priming of CD8+ T cells, even when rendered non-infectious and speculate that this might also be the case for other viruses. Importance: The design of viral vectored vaccines is often considered to require a trade-off between efficacy and safety. This is especially the case for vaccines that aim to induce killer (CD8+) T cells, where there is a well-established dogma that links infection in vaccinated individuals with effective induction of immunity. However, we found that some proteins of vaccinia virus generate strong CD8+ T cell responses even when the virus preparation was inactivated by heat prior to administration as a vaccine. We took advantage of this finding by engineering a new vaccine vector virus that could be used as an inactivated vaccine. These results suggest that vaccinia virus may be a more versatile vaccine vector than previously appreciated and that in some instances safety can be prioritized by the complete elimination of viral replication without a proportional loss of immunogenicity.

Direct Priming of CD8+T Cells Persists in the Face of Cowpox Virus Inhibitors of Antigen Presentation

“…Finally, we wanted to show that CPXV12 + 203 do not impede the supply of antigen from infected cells for cross priming by DCs. To do this, we used a well-characterized in vivo cross priming experiment in which MHC-mismatched cells were infected with MVA or M-CPX12 + 203 in vitro and then heated to inactivate residual virus before being used to immunize mice ( 17 , 19 , 38 ). We then examined CD8 + T cell responses to a set of VACV epitopes that can be cross primed in this setting ( 17 , 38 ) and found that cells infected with either virus elicited a similar-sized response to all specificities ( Fig.…”

Human memory T cell dynamics after aluminum-adjuvanted inactivated whole-virion SARS-CoV-2 vaccination

“…Nevertheless, the nature of antigen has a significant impact on T cell-mediated immunity 58 . Inactivated whole-virion vaccines provide a complex antigenic structure which enhances antigen presentation and T cell responses 59 . Even though there is no consensus in the literature, alum can also positively influence DC maturation and/or increase the costimulatory signals for T cell-mediated immunity 53 .…”

An increase in glycoprotein concentration on extracellular virions dramatically alters vaccinia virus infectivity and pathogenesis without impacting immunogenicity

“…Surprisingly, vMC021L-HA induced strong T CD8+ responses, a hallmark of VACV that spreads effectively [76]. However, immunization with inactivated VACV virions can induce an effective T CD8+ response [81], so it is possible that the production of EV by cells infected at the site of immunization produces sufficient virus particles to reproduce, or even PLOS PATHOGENS exceed this immunogenicity. In addition, when one considers the increased immune response generated by vMC021L-HA compared to vΔF13L, it seems likely that the slight increase in spread (Fig 3) may have been enough to elicit a robust immune response.…”